Qing-feng Sun

Quantum transport theory for nanostructures with Rashba spin-orbital interaction

We report on a general theory for analyzing quantum transport through devices in the metal-QD-metal configuration where QD is a quantum dot or the device-scattering region which contains Rashba spin-orbital and electron-electron interactions. The metal leads may or may not be ferromagnetic, and they are assumed to weakly couple to the QD region. Our theory is formulated by second quantizing the Rashba spin-orbital interaction in spectral space (instead of real space), and quantum transport is then analyzed within the Keldysh nonequilibrium Greens function formalism. The Rashba interaction causes two main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent phase factor in the coupling matrix elements between the leads and the QD, and (ii) it gives rise to an interlevel spin-flip term, but forbids any intralevel spin flips. Our formalism provides a starting point for analyzing many quantum transport issues where spin-orbital effects are important. As an example, we investigate the transport properties of a Aharnov-Bohm ring in which a QD having a Rashba spin-orbital and electron-electron interactions is located in one arm of the ring. A substantial spin-polarized conductance or current emerges in this device due to the combined effect of a magnetic flux and the Rashba interaction. The direction and strength of the spin polarization are shown to be controllable by both the magnetic flux and a gate voltage.

Gate-controllable spin battery

We propose a gate-controllable spin-battery for spin current. The spin battery consists of a lateral double quantum dot under a uniform magnetic field. A finite dc spin current is driven out of the device by controlling a set of gate voltages. Spin current can also be delivered in the absence of charge current. The proposed device should be realizable using present technology at low temperature.

Double quantum dots: Kondo resonance induced by an interdot interaction

We show that through an interdot off-site electron correlation in a double quantum-dot (DQD) device, Kondo resonances emerge in the local density of states without the electron spin-degree of freedom. We identify the physical mechanism behind this phenomenon: rather than forming a spin singlet in the device as required in the conventional Kondo physics, we found that exchange of electron position between the two quantum dots, together with the off-site Coulomb interaction, are sufficient to induce the Kondo resonance. Due to peculiar co-tunneling events, the Kondo resonance in one dot may be pinned at the chemical potential of the other one.

Spin-current-induced electric field

Co-59 nuclear magnetic resonance (NMR) studies have been made to clarify the spin-state and local magnetic properties of Nd1-xSrxCoO3 (0less than or equal toxless than or equal to0.5). Three different types of Co NMR spectra are obtained. The parent NdCoO3 exhibits only one type of a resonance line associated with the low spin state of Co3+ ion. The temperature dependence of spectra indicates gradual transition from low spin (T=0 K) to intermediate spin state. A similar feature has been observed in sample with x=0.05. In the range 0.1less than or equal toxless than or equal to0.2, the spectra consist of two components of resonance lines. The first component is similar to that obtained in NdCoO3, while the second one (much weaker) has been attributed to Co3+ ions (also having low spin) surrounded by few Co4+ ions among its LS/IS Co3+ neighbors. For the samples 0.3less than or equal toxless than or equal to0.5, only the signal of the second type is observed; the first type becomes too broadened to be detected. This signal remains almost unchanged down to 1.5 K, as has been observed in Nd0.7Sr0.3CoO3. Also in the above sample, a very broad signal from Co ions with average internal field of 13 T appears at 1.5 K. This signal is ascribed to the ferromagnetic metalliclike parts of the sample. These data are consistent with the possible phase separation scenario in doped cobaltites.

Hamiltonian approach to the ac Josephson effect in superconducting-normal hybrid systems

The ac Josephson effect in hybrid systems of a normal mesoscopic conductor coupled to two superconducting (S) leads is investigated theoretically. A general formula of the ac components of time-dependent current is derived, which is valid for arbitrary interactions in the normal region. We apply this formula to analyze a S-normal-

Parametric quantum spin pump

S

Microwave-induced π-junction transition in a superconductor/quantum dot/superconductor structure

system where the normal region is a noninteracting single-level quantum dot. We report the physical behavior of time-averaged nonequilibrium distribution of electrons in the quantum dot, the formation of Andreev resonance states, and ac components of the time-dependent current. The distribution is found to exhibit a population inversion; and all Andreev resonance states between the superconducting gap

Probing spin states of coupled quantum dots by a dc Josephson current

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Correlated two-electron transport: A principle for a charge pump

carry the same amount of the current and in the same flow direction. The ac components of time-dependent current show strong oscillatory behavior in marked contrast to the subharmonic gap structure of the average current.

ac Josephson effect in resonant tunneling through mesoscopic superconducting junctions

We investigate a nonadiabatic parametric quantum pump consisting of a nonmagnetic scattering region connected by two ferromagnetic leads. The presence of ferromagnetic leads allows electrons with different spins to experience different potential landscapes. Using this effect we propose a quantum spin pump that drives spin-up electrons to flow in one direction and spin-down electrons to flow in opposite direction. As a result, the spin pump can deliver a spin current with a vanishing charge current.